The present invention relates to rechargeable electrolytic battery cells comprising polymeric film composition electrodes and separator elements, and metallic foil or mesh current collectors, which are typically laminated under heat and pressure to form a unitary battery cell structure. In particular, the invention provides a simple and economical method of preparing such separator element films or membranes which are highly porous and thus capable of absorbing and retaining substantial amounts of electrolyte solution even after high-temperature lamination processing to thereby provide high ionic conductivity and cycling stability over long periods of rechargeable battery storage and use in a broad range of temperatures.
Electrolytic battery cells particularly suited to use of the present invention include Li-ion intercalation cells of the type described in U.S. Pat. No. 5,460,904 which have preferably comprised separator elements such as described in U.S. Pat. No. 5,418,091, the disclosures of which patents are incorporated herein by reference. Such cells are fabricated of respective positive and negative electrode elements comprising finely-divided active materials, such as lithium-source LiMn2O4 and carbon, dispersed in a polymeric matrix and formed into flexible layers or membranes. These elements are laminated to an interposed electrically-insulating separator membrane, usually comprising a similar polymeric material, which will ultimately contain a uniformly distributed organic solution of a lithium salt to serve as an electrolytic, ion-conducting bridge between the electrodes and enable the intercalation of Li-ions flowing reversibly to and from the active materials of those electrodes during the charge and discharge cycles of the battery cell. Finally, to facilitate the concomitant flow of electrons in the battery cell circuit, each of the positive and negative electrode elements has an associated current collector element which also serves as a terminal base for the attachment of a conductor leading, in use, to a utilization device.
In the fabrication and use of these earlier-described battery cells, there were employed separator membrane element compositions comprising a polymer-compatible plasticizer compound which was in part removed from the finished cell separator component in preparation for activation by the addition of electrolyte solution. These polymeric separator compositions, as well as the similar polymeric battery electrode compositions, were particularly unique in that removal of the incorporated compatible plasticizer, typically by extraction with a polymer-inert solvent, did not create a porosity in the polymeric matrix, but rather preconditioned such matrix for ready absorption of activating electrolyte.
Yet earlier polymeric battery cell separator elements which, on the other hand, relied on matrix porosity to promote electrolyte absorption were described in U.S. Pat. No. 3,351,495. Preparation of such separator elements did, however, rely upon a similar solvent extraction operation to physically remove from the solidified composition an incompatible, so-called xe2x80x9cplasticizerxe2x80x9d, component along with entrained inert filler particles in order to achieve the resultant porosity.
In accordance with the present invention, battery separator membrane elements are provided which comprise a degree of mesoporosity capable of yielding improved electrolyte solution absorption and resultant high ionic conductivity, yet are prepared without reliance upon time-consuming and expensive extraction operations. On the contrary, the present battery cell element preparation entails merely an simple element membrane coating or casting process which forms the preferred porosity by differential evaporation of coating composition fluid vehicle components.
The present invention provides a method of preparing mesoporous polymeric matrix layers or membranes which serve equally well as separator and electrode elements in laminated Li-ion intercalation battery cells. Such membranes are prepared in a simple coating or film-casting operation utilizing a polymeric composition comprising a combination of polymer-solvent and -nonsolvent coating vehicle components which, due to respective evaporation rates, results in a fine dispersion of the less volatile nonsolvent throughout the solidifying polymer matrix layer during the primary solvent evaporation operation. The ultimate evaporation and diffusion of such nonsolvent component from the polymer matrix after the latter has gelled or solidified as a result of the evaporation of the solvent component yields the mesoporous matrix structure which will readily absorb electrolyte solution to provide a high level of ionic conductivity in the battery cell.
In addition to the matrix polymer and coating vehicle mixture, the coating composition preferably comprises a finely-divided inert filler, such as silica, the particles of which are initially uniformly dispersed in the coated layer, but which apparently migrate in part to concentrate as agglomerates in the dispersed nonsolvent vehicle droplets during the gelling of the matrix film. Thus, the filler not only imparts structural strength to the final layer in its uniform dispersal, but also provides support to maintain the open structure of the mesopores after diffusion of the nonsolvent vehicle and further extends additional such support during the compressive tendencies of the thermal lamination operation which later joins the separator and electrode elements into the unitary battery cell structure.
Complementary electrode coating compositions comprise, along with the matrix polymer and solvent/nonsolvent mix, respective electrolytically active components, such as powdered carbon and intercalation compound, e.g., a LiMn2O4 spinel. Although these active components are dispersed in the coating compositions in finely-divided form, they do not significantly interfere with the mesopore formation, since their particles are normally in the range of an order of magnitude larger than the mesopore voids.
After completion of the formation of mesoporous separator and electrode elements, the thermal lamination of the respective electrode elements with metallic grid or foil current collectors followed by lamination of those electrode subassemblies with an interposed separator element are substantially as described in the above-incorporated patent specifications. Completion of battery cell fabrication includes activation by application and mesopore membrane absorption of electrolyte solution, and final packaging, in the usual manner.